1. Field of the Invention
The invention relates to a method and an apparatus for detection of mechanical defects in an ingot section composed of semiconductor material.
2. Background Art
In microelectronics, wafers which are composed of a semiconductor material are used as substrates for the production of microelectronic components. Suitable materials are, for example, II/VI compound semiconductors, III/V compound semiconductors or element semiconductors such as germanium or the particularly useful silicon.
The semiconductor wafers are produced by first of all cutting a single-crystal semiconductor ingot into ingot sections with a length of several centimeters up to several tens of centimeters. These ingot sections are then cut into thin wafers with a thickness of about 1 millimeter. Single-crystal semiconductor ingots are produced either without the use of a crucible by means of the so-called float-zone (FZ) process, or by means of the Czochralski crucible-pulling method. Particularly in the case of the Czochralski crucible-pulling method, it is possible for gas bubbles to become trapped in the growing semiconductor ingot. These gas bubbles represent gas-filled cavities in the form of bubbles in the semiconductor ingot, and may have diameters from about 10 μm up to about 10 mm. These gas bubbles are in some cases cut into when the semiconductor ingot is being cut into wafers, so that they are visible on the surface of the semiconductor wafers. Defective semiconductor wafers such as these are segregated before delivery, and are not used for the production of microelectronic components.
Others of the gas bubbles are, however, not cut into during the cutting process, so that the gas bubbles remain as small cavities in the affected semiconductor wafers, although no defect is externally visible. If semiconductor wafers such as these are used for production of microelectronic components, then, depending on their position in the semiconductor wafer, the cavities can lead to failure of individual components, thus reducing the yield from component manufacture.
In order to avoid this, a test method has been used according to the prior art for semiconductor wafers composed of silicon, by means of which each individual completely processed semiconductor wafer is checked for the presence of cavities, before it is delivered and is used for production of components. This method is based on the illumination of one end of the semiconductor wafer with infrared radiation, and the measurement and imaging of the transmission, that is to say of the intensity of the transmitted radiation, on the other end of the semiconductor wafer. Infrared radiation is transmitted through the semiconductor material, with the light being refracted on the boundary surface of a cavity, leading to reduced transmission. This method can be used only for semiconductor materials through which infrared radiation can pass.
This method is applied to surfaces with little roughness, in order to avoid severe light scattering on the surface, and thus reduced transmission. This means that the semiconductor wafers cannot be examined directly after their production by cutting of the ingot sections, but only after further processing steps which smooth the surface, and in the extreme only after they have been polished at the end of the production process. Semiconductor wafers with cavities therefore have to pass through an unnecessarily large number of processing steps before they can be segregated and rejected. However, earlier segregation would be desirable, in order to avoid the costs associated with the processing of defective semiconductor wafers.
The previously described test method also is relatively costly, since it must be carried out on each individual semiconductor wafer. Furthermore, the described method is subject to further restrictions relating to the dopant content, since the light is absorbed by the charge carriers which are released as the dopant content increases, thus greatly reducing the transmitted light intensity.
An ultrasound test method is also known in the prior art, by means of which various mechanical defects are detected in different materials. Until now, the imaging of defects has been restricted to worksection thicknesses of a few millimeters, because the sensitivity of the method decreases at greater depths.
Scanning ultrasound microscopes in which a sample is scanned two-dimensionally by means of ultrasound and in which the sound waves that pass through or are reflected are processed in order to produce an image from them are known from the prior art, for example from DE2504988A1, and international patent application WO01/86281A1 discloses a scanning ultrasound microscope which produces three-dimensional images of a sample. In this case, the images are produced non-destructively, thus resulting in information about the internal structure of a sample. However, the prior art described above is not designed for high-speed data recording of the samples to be examined and for measurement of ingot sections with a length of up to 100 cm. Furthermore, the apparatuses according to the prior art have a limited throughput.